A disc drive assembly usually comprises one or more discs with information stored on annular tracks closely packed together to increase data density. The discs are fastened to or supported on a hub and are rotated at high speed to decrease the time it takes to obtain data via a sensing head which is substantially stationary with respect to the rotation of the disc. The head shifts radially across the tracks to sense data on different tracks.
Disc drive manufacturers and computer manufacturers typically establish standards for vibration and shock resistance for hard disc drives to be used for data storage. The standards may be even more stringent for disc drives intended for use in portable or lap top computers, or other difficult environments. Vibration and impact acceptance evaluation which qualify disc drives and the parts used therein for use in computers are frequently conducted by placing the drive to be evaluated on a vibration table, subjecting the drive to vibrations of varying frequency and amplitude while the drive is operating. The performance of the drive is monitored to determine the frequency and amplitude of the applied vibrations which cause errors in seeking and/or track following. Seek and/or track following errors often result in delays in reading and/or writing data, and disc drives which are sensitive to applied vibrations of too low a frequency or amplitude may fail acceptance evaluation.
One effect of vibrations applied to a disc drive, and a cause of errors of seeking or track following is mechanical off-tracking, i.e. an unintended physical movement of the heads with respect to the disc or discs. Mechanical off-tracking may be caused by movements of the various structural components of the spindle motor, causing the disc to tilt or wobble out of a plane normal to the axis of the motor spindle. Thus, improving the ability of a hard disc drive to withstand applied vibrations by improving the stability and shock absorbing ability of the spindle motor is a continuing goal of motor manufacturers.
However, among the further criteria imposed on hard disc drives in addition to resistance to vibrations is compactness, low weight, low power consumption, and ease of manufacturing. All of these criteria are important to computer manufacturers selecting a disc drive for use in a specific computer for a specific application. Thus, while reducing sensitivity of the spindle motor to applied vibrations is important, these other criteria must also be taken into account. Further, the resistance to applied vibration depends in part on the internal operating vibrations experienced by and absorbed by the spindle motor, because the applied and internal vibrations may act together under certain circumstances resulting in sufficient vibration to cause errors in seeking or track following.
In designing spindle motors for disc drives, all of the above must be taken into account. The spindle motor itself is generally comprised of three major elements; the stator, the rotor and the spindle. Thus, the objective is to dampen the vibrations applied to or created between any one or more of these pieces so that any shock or vibration applied to a part of the spindle motor is not transmitted through the rotating hub which is supported on the rotor and in turn supports the discs for rotation.
A further consideration is that spindle motors are generally separately assemble and then incorporated into the base of the disc drive. It is important that any vibration not be transmitted between the base and the spindle motor where it could create noise or displacement of the transducer relative to a target track.